1. Field of the Invention
The present invention relates to the field of fluid flow systems, and more particularly but not by way of limitation, to a method and apparatus for improving throttling characterization of industrial burners.
2. Discussion
In the control of fuel and air mixtures to industrial burners, furnaces, boilers and the like, there is a need to maintain proper proportions of the fuel and air over widely varying firing conditions from low fire to high fire. This requirement is driven by the desire to achieve maximum heat output while maintaining minimum fuel input. Although ideally these fuel and air mixtures comprise equivalent proportions of fuel and air at each firing rate, fuel can burn in a wide range of excess air levels. However, discrepancies between the ideal air amount and the actual air amount result in inefficient burn. For example, to much air will result in a flame that is cooled because the heat from the flame will be used to warm the excess air surrounding it. On the other hand, to little air will result in incomplete combustion which produces carbonization and high unburned fuel emissions. To assure complete combustion without over-cooling the flame, combustion is generally carried out with about 10% more air than theoretically required. The process in which this fuel to air ratio is held constant over varied firing rates is called characterization. Those skilled in the art will understand that maintaining the optimum ratio between fuel flow and air flow as the burner drives from a low firing rate to a high firing rate is difficult.
Conventional throttle mechanisms utilize throttle plates or shutters to regulate the amount of air which enters the burner. For example, U.S. Pat. No. 4,197,076 to Viger teaches use of a throttle plate of the butterfly valve type to control the air flow into the blower. An impeller is used to draw air into an air scoop and move the air through the burner system. The air scoop has an air intake opening with an adjustable shutter positioned across the opening. The shutter is adjusted to vary the amount of air entering the system.
U.S. Pat. No. 4,230,499 to Binasik, et al. teaches use of dual throttling plates across the entrance of an air intake chamber. The throttling plates are mounted on shafts which are interconnected by spur gears so that the shafts rotate in unison. A lever arm is fastened to one of the shafts. A connecting rod connects the lever arm with an output lever from an actuator to provide a throttle control for air entering the air chamber.
U.S. Pat. No. 4,375,952 to Vosper, et al. also teaches use of dual throttling shutters to regulate the amount of oxygen supplied to the flame in a variable rate burner. The air intake duct is provided with shutters such as a pair of vanes rotatably mounted on spaced shafts which are pivoted from the exterior of the duct via a mechanism so that more or less air can be admitted to the burner depending upon the load under which the burner operates at any given moment.
U.S. Pat. No. 4,595,355 to Garrelfs, et al. teaches an adaptation of the standard shutter system described above by implementing a rotary shutter. Draft control baffles are positioned over the air inlets of the burner. Air flow into the burner is regulated by rotating the baffles to adjust the size of the air inlet opening.
In the case of a butterfly valve throttle plate such as the ones described above, air flow is poorly characterized. Although these types of throttle systems are generally actuated by a mechanical motor which is attached to the shutters through linkage assemblies, they have limited adjustability to achieve adequate characterization. Before operation, the linkage assemblies are manually adjusted based on the stoichiometric requirements for a particular fuel input rate. During operation, in response to system demands, the motor can actuate the linkages and adjust the amount of air which moves past the shutters into the combustion chamber. Although there is sufficient air to allow optimum combustion at a single firing rate, the fuel to air ratio can not be optimally maintained from the low end to the high end of the firing rate range. Thus, these devices are optimized over a narrow firing rate range. If a different firing rate is desired, the linkages must be manually re-adjusted.
A number of prior art devices are known for varying inlet fuel valuing and inlet air venting in response to monitored signals, such as temperature and stoichiometric parameters, over a varying firing range. As pointed out in numerous prior art publications, and as is known by persons of ordinary skill in this field, each industrial burner application will have numerous variables which take each such installation beyond the range of prediction, and thus require that each such installation be provided with the capability of tailoring its characterizing controls to its peculiarities over the range of its firing usage. Several prior art characterizing, or proportioning controls are as follows.
U.S. Pat. No. 2,286,173 to Maxon teaches a valve to proportion air and fuel to industrial burner systems in which an air gate is journaled in an air passage bore and is rotatable by a segmental arm structure. A spring loaded stemmed valve (biased closed) is supported in a fuel inlet bore. A push rod is supported to engage the stemmed valve and is operable to open same as it is pressed against the stemmed valve. Actuating means are disposed to actuate the push rod by the arm structure which supports a series of individually adjustable threaded pins carried by the arm structure to variably depress the push rod along the arc of the arm structure to vary the air-fuel ratio. A flexible strip is disposed between the push rod and the threaded pins to facilitate engagement therewith.
U.S. Pat. No. 2,315,171 to Voorheis teaches an adjustable valve in which a series of adjusting screws provide a path to selectively depress a roller supported by an operating handle connected to a valve rod which is spring biased. The object of the invention is to move the valve control element through a predetermined sequence of valve settings.
U.S. Pat. No. 1,525,052 to Spotz teaches an adjustable cam surface and a cam follower. The shape of the cam surface is determined by a plurality of adjustable struts which connect it to a carrying frame.
U.S. Pat. No. 4,932,274 to Jones teaches a characterizing linkage assembly which comprises a cam comprising an adjustable curvalinear cam band which is rotatable by a drive shaft. A follower rod is urged against the cam band. An actuator arm attached to air intake shutters is attached to the follower rod. The curvalinear shape of the cam band can be varied by manipulating adjustment screws located along the surface of the curve. Rotation of the drive shaft in response to a change in firing rates imparts movement to the cam band, the follower rod, the linkage actuator arm and thus the intake shutters. In this way, the path of the follower rod can be altered, allowing the fuel to air combustion ratio to be held relatively constant as the burner is throttled between firing rates.
It would be desirable to provide a throttle assembly which could achieve optimum characterization between air and fuel as a burner is throttled between firing rates, but require very few moving parts. Such throttle assemblies would be less likely to succumb to mechanical problems and would be much less operator intensive.